I've never seen this before. There are two of us travelling together, and I'll pick the aisle seat, then attempt to pick the window seat, but I can't. As soon as I select the aisle seat, the previously available window seat becomes unavailable. I have tried this multiple times and always get the same result.

Anyone seen this before? Any way around this besides booking separate reservations for each person?

(Booking separate reservations not worth it as I get free seat selection by virtue of my Delta Elite status but if we booked separate reservations my companion would have to pay for advance seat selection.)